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At $dayjob I'm working on a project where the home-grown error reporting module occasionally warns Use of "goto" to jump into a construct is deprecated

So now I'm trying to find out how exactly goto LABEL works, in order to assess whether the whole error reporting module needs to be rewritten. And I found no documentation.

The closest I found was "goto only searches the immediate call stack for labels", but this alone doesn't explain how "jump into a construct" can happen at all -- if it only searched the caller's scope and the caller's caller's scope etc. for the label, it would never find label from inside a construct that's not in dyanmic call chain.

Some curious experiments:

$ perl -wE 'if (0) { label: say 1 }; goto label' Can't find label label at -e line 1. $ perl -wE 'if (1) { label: say 1;}; goto label' Use of "goto" to jump into a construct is deprecated at -e line 1. 1 Use of "goto" to jump into a construct is deprecated at -e line 1. 1 ^C # aborted, because it loops

So apparently, the construct containing the label has to have been executed once.

This works:

#!/usr/bin/perl use strict; use warnings; use 5.014; sub f { my $arg = shift; if ($arg == 0) { label: say 'here'; } elsif ($arg == 1) { # don't execute a branch with 'label:' } else { goto label } } f($_) for 0..2; __END__ here here

Whereas this one dies:

$ perl -wE 'sub g() { label: say "here" }; g; goto label' here Can't find label label at -e line 1.

I have trouble finding a clear mental model that can help me to explain all of this behavior. Who can help?


In reply to How does 'goto LABEL' search for its label? by moritz

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